A. Field of the Invention
The present invention relates to hydraulic cementitious compositions, including concrete, stucco, and mortar, which contain reinforcing fibers of polyacrylonitrile having a molar concentration of acrylic nitrile monomer units in the range of from about 98 percent to about 100 percent.
B. Description of the Prior Art
Hydraulic cement compositions, such as concrete products, have long been recognized as desirable for construction purposes, especially for use in structures subject to heavy loading, high external or internal pressures, etc. where the weight of the structure is not a critical factor.
A relative weakness of certain hydraulic cement compositions used as structural materials is the propensity of the matrices to crack during drying, setting, curing or when compressed. Thus, it is known to include materials within these compositions for the purpose of mitigating these problems.
The demands placed on fibers which are designed for reinforcement of cement and other hydraulically setting materials are extremely high. For instance, the physical fiber data should agree with the physical data of the cement in important characteristics. It is known that cement exhibits a certain brittleness and can break, for example, at an extension of as little as 0.3%. With respect to the use of reinforcing fibers in cement, it has been shown that fibers have the best reinforcing effect when they oppose a minimal extension of the cement with the greatest force. It must be noted in this regard, however, that the characteristics of fibers treated with an aqueous cement paste can change, and, thus, it cannot be foreseen to what degree such a change can occur. This means that despite good original mechanical values of a fiber, when it is used in the cement, it may not achieve the expected effect if its characteristics change during the hydration process of the cement.
In addition to these described physical characteristics of fibers, it is also important that the fibers be well dispersed in an aqueous cement paste and also remain uniformly distributed therein when additional additives are included within the cement. This characteristic is especially important when the fibers are utilized to form fiber-cement products by drainage methods.
Steel fiber, wire, or rods have been utilized in cement in an attempt to counteract some of these problems. The use of such material is unsatisfactory for a number of reasons. Steel has low elongation even at high tensile stresses and, therefore, is likely to slip within the matrix when the structural member is subjected to loads. In addition, due to the relative stiffness and lack of workability of steel, it is relatively heavy and therefore costly to achieve a given proportionate volume within the matrix.
The use of glass as reinforcement for cementitious materials, such as concrete, has been investigated for many years. Consideration has been given to the use of glass fiber rods to replace steel reinforcement, as well as the use of glass enforcement in the form of fibers, flakes and woven or nonwoven fabrics. Particular attention has been given to the use of glass fibers as a reinforcement for cement or concrete.
A serious problem in employing glass as a reinforcement for cement is the alkaline environment of the inorganic cementitious composition, which is highly deleterious to the glass and results in significant loss of strength in the reinforced cement or concrete products over a period of time, or even in total destruction of the glass reinforcement. Prior art approaches to overcoming the problem of alkali attack of the glass reinforcement have included the use of low alkali-type cements, the use of coatings to protect the glass from the alkali, the use of specialized alkali resistant compositions, and the use of a cation exchange material to change the inorganic alkaline binder to a form that does not attack to the glass. These approaches have not been entirely successful due to the limited availability and expense of the specialized materials required for these approaches.
The use of organic fibers admixed in cement or concrete for strengthening purposes is well known. For instance, U.S. Pat. No. 3,591,395 discloses the use of fibrous reinforcing elements formed from a stretched and then fibrillated plastics film material which is preferably a polyolefin film which has a length of at least 2 cm. and, more preferably, a length of 5 to 8 cm. A problem associated with such reinforcing elements is that fibers containing polyolefins are not particularly effective in reinforcing cementitious material and, due to the long fiber length of these fibers, large quantities of the reinforcing material are required which increases the expense of using such reinforcing fibers.
Other organic fibers have been proposed for use in reinforcing a cementitious composition. For instance, wool, cotton, silk, polyamide, polyester and polyvinyl alcohol fibers have been suggested. None of these fibers have proven entirely satisfactory as reinforcing materials in a cementitious composition.
Polyacrylonitrile fibers have been proposed, such as in U.S. Pat. No. 4,414,031, for use in reinforcing a cementitious composition. These fibers are subject to the same limitations, with respect to cement reinforcement, as the previously described fibers. It has been previously suggested that, to be effective for cement reinforcement, polyacrylonitrile fibers require a high strength (at least 5.7 g/denier), high modulus (e.g. 113 to 226 g/denier) and low extensibility (up to 15 percent).
Accordingly, a need exists for fibers in which can be effectively and economically used as reinforcements in hydraulically setting, structural materials, such a cementitious composition.